The Magazine for Underwater Professionals
Survey operations in the historic Dubrovnik Old Town Port
In August 2017 and November 2018, on behalf of the Dubrovnik Port Authority, Croatia, and the Croatian Institute of Marine Heritage (Ars Nautica), GEOmar – a survey company from Split, Croatia – conducted survey operations in Dubrovnik Old Town Port.
The operations were carried out with the following objectives:
Dubrovnik Old Town Port is a medieval port located at the eastern part of the city of Dubrovnik. The port got its present day look in the 15th century and it incorporates two breakwaters: breakwater ‘Porporela’ in front of St John fortress, and the ‘Kaše’ breakwater going perpendicular across the port bay. The Kaše breakwater is a remarkable monument of Renaissance civil engineering. It was built in four stages between 1486 and 1514, in dry docks. The first, smaller part was built in 1486, while the other, somewhat bigger, in 1487, to be joined in 1498, and expanded to the north in 1514. The breakwater was built in a submerged and later dried out dock. The latter was framed with sheet piles (a layer of joint wooden posts), while the bottom consisted of two layers of bars upon which the breakwater is bedded. The breakwater was built from large stone blocks, joined by hydraulic mortar composed of red clay and lime, with no pozzolanic additives. Stone blocks were first connected with iron and later bronze joints and clamps covered with lead. The central part of the breakwater is ‘cemented’ with lime mortar, red clay and sand, with an addition of crushed stone. Besides protecting the port during bad weather conditions, Kaše also had a major part in protecting the city. From the southern part of it, big chains would be put across to the St John fortress, locking the entrance.
Today, the breakwater is in a dilapidated condition and in desperate need of repair. The Development Agency of the City of Dubrovnik acknowledges that it is an interesting cultural-historical asset that requires reconstruction, but first needs to be thoroughly researched. A project started by the Development Agency of the City of Dubrovnik will research the historical and geo-archaeological significance of Kaše, located squarely in the middle of Dubrovnik’s iconic harbour. The project will fall under the auspices of APPRODI (From Ancient Maritime Routes To Eco-Touristic Destination), and be funded by INTERREG V-B of the Adriatic-Ionian Transnational Cooperation Programme. It will be the first time underwater archaeological research is carried out on the historic Dubrovnik breakwater. Modern hydrographic, geological and geophysical techniques will allow researchers to study the entire old city harbour in greater detail, and to create a historic timeline of its development without endangering the site itself. Routine inspections are the key to the long-term care and success of any infrastructure. The importance of these inspections relates to the long-term health of the structure, due to the effect that the sea has on construction materials.
In 1979, the old city of Dubrovnik, along with its famous city walls that protected this port as well as other parts of the city, joined the UNESCO World Heritage List. Surveying in such an environment brings with it some challenges that need to be overcome to make the survey successful. The survey was undertaken in August when Dubrovnik’s imposing city walls are one of the city’s top attractions. Especially during the summer months, Dubrovnik attracts hundreds of thousands visitors each year, and therefore there are numerous walking tours as well as sightseeing boat tours that pass just through the survey area. Also, regular boat lines to nearby places like Cavtat, Mlini and the island of Lokrum depart from the Old Town Port. It is important to note too that the air temperature in Dubrovnik reaches 35°C in August. The logical solution was to start early and work while others were sleeping or still drinking morning coffee.
After solving the issue of when to survey, there were still logistical problems for which it was necessary to find technical solutions: firstly, the port at present is a safe haven for many small private boats owned by local citizens and therefore boat manoeuvring is limited; secondly, there are some very shallow areas inside the port which cannot be surveyed by a multibeam echosounder; and thirdly, a nearby bathing area is fenced with buoys.
To solve these three problems and provide the client with a complete overview of the state of the coastal and underwater part of the structure, the survey was conducted by using the following methods:
The first step taken was drone aerial mapping and collecting high-resolution topographic data. It is an important part of the survey process because the coastline changes over the years, for multiple reasons (undermining, erosion, accumulation by rivers and construction of a new port, for example). Flying the UAV at low water means that the resulting aerial orthophoto can be analysed for obstructions and used as the shoreline edge of the survey for the MBES, improving efficiency further.
In order to meet IHO Special Order standards, GEOmar used the latest state-of-the-art multibeam echosounder from Teledyne Marine, USA. The Teledyne RESON SeaBat T20-P has a range of features that are ideally suited to high-quality shallow-water data acquisition. The SeaBat T20-P was operated at 400kHz, with 512 beams in every ping. The boat was a small, seven-metre long Colnago 20, with a shallow draught and a 100HP outboard engine that enables surveying in shallow coastal sea areas just like this one. The size of the boat dictated the need for a small portable unit, and therefore the T20-P, with its portable, splash resistant suitcase design, was choosen.
The sonar head was pole mounted over the starboard side with a head tilt of 15 degrees. On this boat, there was the possibility to mount the pole in two positions. If surveying a shallow area with the possibility of obstacles in the water, the pole was mounted in the first position, with the sea level 20-30 centimetres above the head. The second position was used when surveying a deeper area where there is no danger of running aground with the sonar head. In this position, the sea level was 50-60 centimetres above the sonar head.
The portable SeaBat T20 sonar processor is a small, compact, portable and water resistant unit, which was placed in the hull of the boat with other system components. To form a complete mapping system, the multibeam echosounder system was connected to the position, heading and motion sensing instrument in order to collect geo-referenced soundings. An Applanix, Canada, POS MV motion sensor was used to provide accurate vessel attitude, heading, heave, position and velocity data of the survey vessel, allowing geo-referencing and motion compensation of all the data obtained by the SeaBat. The motion sensor was installed directly on the sonar head bracket and the GNSS antennas were installed on a horizontal pole above the sonar head installation (as shown in the picture). The MBES and positioning system were operated from an external PC using the Sonar UI and POS View software applications.
Teledyne PDS software was used to interface the position, motion and multibeam data, as well as for acquisition, initial processing, and the final cleaning and selection of survey data sounding. The final chart containing depths, contours and profiles was created in Teledyne PDS by using the Plot option and then exported to DXF, which was then delivered to the client.
Depths in the survey area ranged from 0-15 metres and, due to heavy traffic within the harbour, survey lines were adjusted on-the-fly. The combination of tilted multibeam heads and the possibility of switching between equidistant and equiangle beam spacing allowed mapping of the whole area in great detail – especially the breakwater Kaše, which was the principal subject of archaeological research, and some tricky sections such as areas below a group of small vessels.
In the shallowest parts of the survey area (less than 1.5 metres depth), as well as the above water portions, conventional topographic survey techniques like GNSS and total station were utilised. Combining these two types of survey, a 0.2-metre digital terrain model (DTM) of the seabed was created. This was small enough to show waste material such as car tires and plastic, as well as some stone ledges amongst other items of interest. The standard deviation of the DTM is 0.03 metres, which gives a good indication of the overall survey quality and confidence in the results. This ultra-high resolution DTM enabled geologists, archaeologists and divers to define areas of interest for archaeological research and sub-bottom profiling (to identify and measure various marine sediment layers).
While surveying this area, also mapped were nearby submarine cables and pipelines, which the client found very interesting because of the lack of any previous hydrographic survey data on these installations.
The final step was to integrate all four surveys into a single study. These combined measurements – linking the topographic and hydrographic surveys together, so that highly accurate coastal engineering decisions can be made from the data – and successive surveys can be assessed to look at temporal variations.
Using the final combined dataset, GEOmar was able to provide product outputs in hard copy or electronic formats, to suit any requirement. The final products include: a report, CAD charts, and 3D models. All of the products were supplied using the official coordinate system of the Republic of Croatia.